It has been proposed that two spacecraft might communicate one with another via laser beams. Each spacecraft would be provided with a transmitter/receiver terminal able to transmit a modulated laser beam to another spacecraft and to receive a modulated beam sent from that other spacecraft.
The terminals must then be accurately tracked one onto another using movable mirror mechanisms and in addition the transmit laser beam must be aligned and pointed to a position where the other spacecraft will be when the beam reaches it as the two spacecraft, and hence terminals, may be moving relative to one another. Thus each terminal may comprise a single telescope assembly and common pointing mechanisms (movable mirror assemblies for example) for the transmitter and receiver parts of the terminal. By use of the pointing mechanism(s), the transmit laser beam may be maintained pointing at the other spacecraft or, since the two will be moving relative to one another, at the position where the other spacecraft will be when the laser radiation arrives at the other spacecraft (using appropriate look-ahead mechanisms) while the receiver has to be kept tracked onto the receive beam in the face of disturbances such as background radiation, vibration of the spacecraft frame, and other transient features such as thermal expansion and contraction of the terminal due to heat radiation from the sun.
These disturbances are variable and this means that the pointing mechanism (point ahead mechanism) for the transmit beam in particular has to be controlled for alignment and/or calibration at least occasionally and it is desirable that this should be done without interrupting communication, preferably automatically. The possibility of maintaining similar control of the receive beam pointing mechanism is also desirable.